1. Field of the Invention
This invention relates to the field of integrated circuits. More particularly, this invention relates to the provision of protection against electrostatic discharge events for integrated circuits.
2. Description of the Prior Art
It is known to provide integrated circuits with mechanisms providing protection against otherwise damaging electrostatic discharge events. Electrostatic discharge events can result from a spike in the voltage at an external pin or pad of the integrated circuit which can feed through to circuit elements within the integrated circuit and permanently damage those circuit elements.
As an example of a mechanism which can provide electrostatic discharge event protection it is known to include a clamp transistor between a power supply rail and a ground rail within an integrated circuit and to control the clamp transistor so that it provides a high capacity current path for any unwanted charge introduced by an electrostatic discharge event to pass to ground bypassing other circuit elements within the integrated circuit which would otherwise potentially be damaged by such a current. Circuit elements which are directly connected to external signal lines of the integrated circuit, such as transmitter and receiver circuits, may be provided with additional protection in the form of electrostatic discharge diodes connected between the external signal line and the power supply rail and/or the ground rail so as to be reversed biased and non-conductive during normal operation but forward biased should a voltage spike occur on the external signal line driving the external signal line out of its normal operating range. The electrostatic discharge diodes can thus provide a path to ground for the unwanted charge and reduce the possibility of damage to the circuit elements directly connected to the external signal line.
A problem with the use of electrostatic discharge diodes is that they increase the capacitance of the external signal line. This can be a problem, such as when the external signal line is a high speed serial data communication signal line connecting to a transmitter or a receiver. Such high speed communication lines are degraded in performance by too much capacitance and some communication standards define maximum amounts of capacitance which should be present. Meeting these communication standards can be difficult whilst also providing an appropriate degree of electrostatic discharge protection through the use of electrostatic diodes.